1. Field of the Invention
The present invention relates generally to systems for maintaining minimum pulse densities in digital communication systems and, more particularly, relates to a system for efficiently maintaining a minimum pulse density without substantially increasing the bit error rate (BER) of the digital communication system.
2. Description of the Relevant Art
Regenerative repeaters for digital communication systems typically require some minimum pulse density to preserve timing information sufficient to regenerate pulses below a specified BER. One example of a minimum pulse density requirement is set forth in the specification for the AT&T T1Transmission System. The T1 specification relates to a data format having frames formed by bytes of eight bits each. The data is transmitted at a fixed rate of 1.544 Mbps. The minimum pulse density constraint requires that no more than fifteen consecutive ZEROs may be generated and that an average pulse density of one pulse in eight be maintained. Other constraints relate to framing bits and signaling information.
Existing systems for maintaining pulse density requirements typically change specified ZEROs in the data stream to ONEs when required to maintain the minimum pulse density.
One obvious drawback to such a ones forcing scheme is an increase in the bit error rate (BER) due to the arbitrary changing of specified bits from a ZERO to a ONE. Various systems for minimizing this increase in the BER exist in the prior art.
One system is disclosed in a patent to Caldwell entitled "Zero Suppression In Pulse Transmission Systems," U.S. Pat. No. 3,924,080. The Caldwell system utilizes the principle of artificial data block definition to avoid forcing ONEs unnecessary to achieving the required pulse density. By avoiding forcing unnecessary ONES the system in Caldwell reduces the increase in the BER.
Another prior art system for maintaining minimum pulse density requirements is disclosed in the patent to Nishiwaki entitled "Digital Data Transmission System," U.S. Pat. No. 4,353,129. Nishiwaki relates to a video pulse transmission system in which transmission of pulse data is effected through a buffer memory. In that system, if too many ZEROs occur in succession, a dummy bit is inserted in the data steam to maintain the minimum pulse density requirement. The system also tags a status bit to indicate the presence of the dummy bit to the receiver. The receiver removes the dummy bit from the data stream to prevent undesired increases in the BER.
The system of Nishiwaki transmits data at a variable rate, depending on the number of dummy bits inserted in the bit stream. Additionally, Nishiwaki is not directed to a word organized data stream as is present in the T1 System and utilizes a significant portion of the bandwidth as overhead.
In some applications, any increase in the BER due to the arbitrary insertions of ONEs into the data stream is unacceptable. Accordingly, a system that maintains minimum pulse density requirements without substantially increasing the BER or overhead requirement is greatly needed in the communications industry.